Barrier Properties of Cr/Ta-Coated Zr-1Nb Alloy under High-Temperature Oxidation

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This work investigate the behavior of Zr-1Nb alloy with Cr/Ta coatings having different crystal structure of Ta during high-temperature steam oxidation in the temperature range of 1200-1400 °C. In some cases, the text must be clarified and there are issues that are not accurate. Better and more evidences for some results are required.  

1. Line 54-81, authors should give brief discussions, companions and conclusions for these reference and induce your point.

2. Why are the thickness of coating ~11 μm and outer layer ~11 μm?

3. Error bars should be added.

4. In Fig.4-6, high magnification SEM image of Ta layer should be added.

5. The SEM of Cr/Ta coatings before oxidation should be added.

6. In section 3.3, the characterizations of OM is same as SEM, please modify them.

Author Response

Dear Reviewer, thank you for your valuable comments that allowed us to improve our article! Below are the responses to the remarks. All corrections in the article are highlighted in green colour.

1. Line 54-81, authors should give brief discussions, companions and conclusions for this reference and induce your point.

Reply: We have taken into account your suggestions. The manuscript has been updated.

2. Why are the thickness of coating ~11 μm and outer layer ~11 μm?

Reply: The total thickness of the whole coating is 11 μm. The outer layer of chromium was ~8 μm, and the Ta interlayer was ~3 μm.

3. Error bars should be added

Reply: Error bars were added.

4. In Fig.4-6, high magnification SEM image of Ta layer should be added

Reply: We obtained high magnification SEM images of Ta layer, but their quality is not high enough to understand the microstructure of the investigated samples. As an example, below is an image of Cr/α-Ta a sample oxidized at 1330 °С for 120 seconds (Figure 1). It can be seen that the enlarged image does not show the structural features of the barrier layer. Figures 5-7 illustrate well the oxidation of the samples depending on the parameters (temperature and time). They are sufficient to reveal the oxidation peculiarities of the samples. For these reasons, we have decided not to add enlarged images of the tantalum sublayer in Figures 5-7.

Figure 1 - SEM image of Cr/Ta coated Zr alloy oxidized at 1330 °С for 120 seconds.

5. 5. The SEM of Cr/Ta coatings before oxidation should be added.

Reply: The SEM of Cr/Ta coatings before oxidation was added.

6. In section 3.3, the characterizations of OM is same as SEM, please modify them.

Reply: Characterisations of OM were modified.

Best wishes,

                                        Dmitrii V. Sidelev

                                        PhD, As. Prof.

                                        Tomsk Polytechnic University

                                        phone: +7-3822-70-17-77-1-2518# (add. 2518)

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The comments are as follows:

1) How the chemical compositions of the alloy and coating material was selected?

2) How the parameters for the coatings deposition was optimized.

3)  An optical micrograph of the coated samples can be incorporated.

4) its 10^-3 Pa. at least of 3×10-3 Pa.

5) check it use of 37 in this sentence, “layer Cr coating (~13.5 mg/cm²) 37after oxidation at 1400 °C”.

6) Explain it, “It is important to note that the coating on the Cr/β-Ta 255 sample completely delaminated, while Cr/α-Ta samples maintained their homogeneity 256 after oxidation at 1200 °C for 2000 s”. 

Author Response

Dear Reviewer, thank you for your valuable comments that allowed us to improve our article! Below are the responses to the remarks. All corrections in the article are highlighted in green color.

1. How the chemical compositions of the alloy and coating material was selected?

Reply: Zirconium alloy Zr-1Nb is a material used to produce fuel cladding for water-water energetic reactors (WWER) due to small thermal neutron capture cross-section, excellent corrosion resistance, good mechanical properties In the Zr-1% Nb alloy, the base is α-phase, which, in terms of texture and anisotropy, brings this alloy closer to pure zirconium. The reasons for alloying zirconium with 1% niobium are following: [Azarenkov N.A., Neklyudov I.M., Voyevodin V.N. Reactor materials – modern status // Journal of Kharkiv National University. Phys. Ser. «Nuclei, Particles, Fields». – 2012. – Vol.1017. – Iss.3(55). – P.4-18.]:

• niobium has a small neutron capture cross-section, so the capture cross-section of the alloy as a whole increase insignificantly;
• the role of harmful impurities in zirconium is significantly altered by 1% niobium improves corrosion resistance;
• niobium effectively reduces the proportion of hydrogen absorbed by the zirconium alloy.

The chromium protective coating was chosen as a good corrosion resistance material in a water steam due to protective Cr₂O₃ scale formed under high-temperature oxidation. Also Cr has good adhesion to Zr alloys, relatively small thermal neutron capture cross-section, and elastic modulus of Cr is about twice that of Zr, which helps to improve the overall stiffness of the coated cladding.

The application of tantalum as a barrier layer is theoretically reasonable due to fact, that Ta does not form intermediate phases nor with Zr, nor with Cr in the considered range of temperature (Isayev, R.; Dzhumaev, P. Interaction of a diffusion barrier from the refractory metals with a zirconium alloy and a chrome coating of an accident tolerant fuel. Nuclear Engineering and Design 2023, 407, 112307. https://doi.org/10.1016/j.nucengdes.2023.112307;). Ta also has a good adhesion to Zr and Cr, acceptable neutron capture cross-section.

2. How the parameters for the coatings deposition was optimized.

Reply: The optimization of deposition parameters for Ta interlayer and main Cr layer was done separately.

• Ta interlayers. To deposit α- and β-Ta, we deposited Ta coatings onto Zr sheets and then analyse its crystal structure by XRD. Firstly, we used single direct current (DC) magnetron sputtering and change a bias potential (from grounded to -300 V) and substrate temperature (150–350 °C). After this, we applied dual direct current (DC) magnetron sputtering with varying a bias potential (from grounded to -300 V) and substrate temperature (150–350 °C).
• Main Cr layer. The deposition mode of Cr coatings was chosen based on previously published results in ref. [Kashkarov, E.; Afornu, B.; Sidelev, D.; Krinitcyn, M.; Gouws, V.; Lider, A. Recent advances in protective coatings for accident tolerant Zr-based fuel claddings. Coatings 2021, 11, 557, https://doi.org/10.3390/coatings11050557].

3. An optical micrograph of the coated samples can be incorporated.

Reply: We assume that the since all microphotographs are separated by oxidation parameters and by differences in tantalum phase, combining all images into one would be inconvenient for the reader. We worry that such an incorporation will make the material more difficult to understand.

4. its 10-3 Pa. at least of 3×10-3 Pa.

Reply: Has been changed according to your remark

5. check it use of 37 in this sentence, “layer Cr coating (~13.5 mg/cm2) 37after oxidation at 1400 °C”.

Reply: It has been corrected.

6. Explain it, “It is important to note that the coating on the Cr/β-Ta 255 sample completely delaminated, while Cr/α-Ta samples maintained their homogeneity 256 after oxidation at 1200 °C for 2000 s”.

Reply: We have taken into account your remark. The OM micrographs analysis showed that the coating of the Cr/β-Ta sample completely delaminated, while the coating of the Cr/α-Ta sample oxidized uniformly maintaining adhesion with the zirconium alloy after oxidation at 1200 °C for 2000 s.  The manuscript text was changed.

Best wishes,

                                        Dmitrii V. Sidelev

                                        PhD, As. Prof.

                                        Tomsk Polytechnic University

                                        phone: +7-3822-70-17-77-1-2518# (add. 2518)

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Title: Вarrier properties of Cr/Ta-coated Zr-1Nb alloy under high-temperature oxidation

Manuscript ID:  metals-2986910

Comments to the authors:

The authors have studied α-Ta and β-Ta barrier interlayers between a Cr layer and a Zr-1Nb alloy substrate at temperatures especially between 1200-1400°C under steam oxidation. A protective layer is required to prevent reactions between Zr reactors or containers and water steam at high temperatures and should be improved.

They describe the sample preparation and the characterization methods used. The authors identify a weight gain higher in comparison to a pure Cr protective layer at 1400°C and they show the oxidation processes by EDX mapping. XRD spectra indicate phase changes. Among other things, the authors conclude, that the corrosion resistance of the Cr/α-Ta-coated Zr alloy samples is better than Cr/β-Ta under all considered conditions and the Ta barrier interlayer deposited between protective Cr coating and Zr-1Nb zirconium alloy can limit Cr-Zr interdiffusion due to deceleration of Cr2Zr phase formation under high temperature oxidation.

The article is well written and comprehensible.

I have only a few minor remarks below:

1.       Can you please indicate the orientation numbering of the x-ray diffraction reflexes in the text or better in all XRD- diagrams .

2.       Figure 1: β-T/Zr à β-Ta/Zr. I think an „a” is missing.

3.       When you want to compare the results of Cr/Ta with the pure protection Cr layer, it could be easier for the reader, when the values of Cr (for instance Figure 2,3) can be found also in the diagrams (e.g. as a marker line). This makes it easy to classify the measured values in comparison.

4.       What is the reason for the large increase in weight of the β-Ta sample at 1400°C?

5.       You have written: “Long-term oxidation is accompanied b by the formation of pores, at the boundaries of which, according to EDS-analysis, oxygen, tantalum and zirconium are localized (Figure 4). Such pores were found only in the samples, which were tested at 1200 °C for 1000 and 2000 s. The mechanism of formation of these pores remains unexplored and more detailed studies are required.” Particularly large pores or cavities are formed when using α-Ta at 1200°C. If oxygen, tantalum,... are involved in the formation of the cavities, shouldn't these materials be visible in the EDX mapping in a correspondingly large size? But the cavities are black. Nevertheless, do you have still a possible explanation for the formation of the cavities (pores) especially in these cases?

6.       On page 13 (in section 3.4.) you explain what can be seen in Figure 10. Figure 10 shows a very nice overview of the development of the X-ray diffraction patterns. But unfortunately, it is difficult to follow the statements. Perhaps you could try to explain the statements mentioned (e.g. "There is only a certain shift in the intensities of the reflections of α-Zr, α-Cr and α-Ta towards smaller 2θ angles, which is due to the increase in the lattice parameters of the materials due to their thermal expansion. The phase transition from α-Zr → β-Zr is observed at a temperature of about 900 °C" ) by creating a separate diagram with selected spectra of the temperature range in addition.

Author Response

Dear Reviewer, thank you for your valuable comments that allowed us to improve our article! Below are the responses to the remarks. All corrections in the article are highlighted in green color.

1. Can you please indicate the orientation numbering of the x-ray diffraction reflexes in the text or better in all XRD- diagrams.

Reply: The orientation numbering of the X-ray diffraction reflexes were added to the XRD patterns.

2. Figure 1: β-T/Zr à β-Ta/Zr. I think an „a” is missing.

Reply: The typo was corrected.

3. When you want to compare the results of Cr/Ta with the pure protection Cr layer, it could be easier for the reader, when the values of Cr (for instance Figure 2,3) can be found also in the diagrams (e.g. as a marker line). This makes it easy to classify the measured values in comparison.

Reply: The values of Cr weight gains were added to the diagrams.

4. What is the reason for the large increase in weight of the β-Ta sample at 1400°C?

Reply: We assumed that the significant increase in weight gain is directly related to the phase transformations of β-Ta during oxidation. β-Ta has a tetragonal structure with a large unit cell (a ≈ 10.2 Å, c ≈ 5.3 Å), whereas α-Ta has a cubic lattice with a much smaller unit cell (a ≈ 3.3 Å). High temperature oxidation leads to β-Ta→α-Ta transformation [Lee, S. L., Doxbeck, M., Mueller, J., Cipollo, M., & Cote, P. (2004). Texture, structure and phase transformation in sputter beta tantalum coating. Surface and Coatings Technology, 177, 44-51, Jin, Y., Song, J. Y., Jeong, S. H., Kim, J. W., Lee, T. G., Kim, J. H., & Hahn, J. (2010). Thermal oxidation mechanism and stress evolution in Ta thin films. Journal of Materials Research, 25(6), 1080-1086]. This transformation results in compressive stresses (strains) in the tantalum layer. In turn, tensile stresses act on the side of the chromium coating, creating new oxygen diffusion pathways, resulting in accelerated oxidation of the zirconium alloy with the protective coating. In the case of α-Ta, no phase transformation occurs during oxidation, which has a positive effect on high temperature oxidation in steam. Thus, the values of corrosion weight gain values of Cr/α-Ta samples are lower than those of Cr/β-Ta samples under the same oxidation conditions

5. You have written: “Long-term oxidation is accompanied by the formation of pores, at the boundaries of which, according to EDS-analysis, oxygen, tantalum and zirconium are localized (Figure 4). Such pores were found only in the samples, which were tested at 1200 °C for 1000 and 2000 s. The mechanism of formation of these pores remains unexplored and more detailed studies are required.” Particularly large pores or cavities are formed when using α-Ta at 1200°C. If oxygen, tantalum,.. are involved in the formation of the cavities, shouldn't these materials be visible in the EDX mapping in a correspondingly large size? But the cavities are black. Nevertheless, do you have still a possible explanation for the formation of the cavities (pores) especially in these cases?

Reply: Thank you for such a good question. Some extra SEM images and EDS with local mapping were taken for proving the composition of the pores. As an example, we have placed an image of a Cr/α-Ta sample (1200 °C, 1000 s) in the response (Fig. 1).

Figure 1 - SEM image of a Cr/α-Ta specimen tested at 1200 °C for 1000 s

According to EDS analysis, it was found that the pores were enriched with tantalum, zirconium and oxygen. A similar pattern was observed in our previous paper on the Mo intermediate layer. The mechanism described in that paper implied that the oxidation results in the formation of a liquid eutectic phase with the release of gaseous products, which leads to the formation of blisters, mainly at the oxide interface [Syrtanov, M.S.; Kashkarov, E.B.; Abdulmenova, A.V.; Gusev, K.; Sidelev, D.V. High-Temperature steam oxidation of Accident-Tolerant Cr/Mo-Coated Zr alloy at 1200–1400 °C. Coatings 2023, 13, 191. https://doi.org/10.3390/coatings13010191]. However, this mechanism does not describe the situation with the tantalum sublayer due to the high temperatures of the tantalum eutectic. The pores do not have similar round shapes, indicating that they are not filled with gas. Perhaps, oxygen can affect on the Ta-Zr eutectic phase temperature, but it should be additionally investigated. At present moment, the samples are being prepared for oxidation at 1200 °C and different temperature exposures to clarify the moment of pores formation. We believe that it will help us to reveal the oxidation mechanism of the Cr/Ta samples.

6. On page 13 (in section 3.4.) you explain what can be seen in Figure 10. Figure 10 shows a very nice overview of the development of the X-ray diffraction patterns. But unfortunately, it is difficult to follow the statements. Perhaps you could try to explain the statements mentioned (e.g. "There is only a certain shift in the intensities of the reflections of α-Zr, α-Cr and α-Ta towards smaller 2θ angles, which is due to the increase in the lattice parameters of the materials due to their thermal expansion. The phase transition from α-Zr → β-Zr is observed at a temperature of about 900 °C") by creating a separate diagram with selected spectra of the temperature range in addition

Reply:Thank you for a great remark! We have changed that according to your suggestion.

Best wishes,

                                        Dmitrii V. Sidelev

                                        PhD, As. Prof.

                                        Tomsk Polytechnic University

                                        phone: +7-3822-70-17-77-1-2518# (add. 2518)

Author Response File: Author Response.pdf